The present invention relates to a process for producing liquid fuels from coal. More particularly, it relates to an improved coal liquefaction process for converting coal to a crude petroleum refinable by conventional petroleum refining techniques to produce gasoline and/or diesel fuel.
The consumption of energy in the United States and in other parts of the world has been rising rapidly, while the ratio of petroleum reserves to consumption appears to be declining. This combined with rising costs for manufacture of gasoline and diesel fuel from coal requires improved technology for producing a suitable refinable, crude petroleum substitute from coal.
Conversion of coal to a synthetic petroleum crude oil product requires three basic steps. First, it is necessary to transform solid coal into a liquid form and second to remove its inorganic mineral (i.e., ash) content. In the third place, sulfur, nitrogen, and oxygen removal is required. In addition, for purposes of economy and maximum efficiency, a coal liquefaction process should be capable of transforming asphaltenes into low molecular weight hydrocarbons.
To produce a reproducible petroleum crude requires that the asphaltenes be hydrogenated and converted to low molecular weight aliphatic, naphthenic, and aromatic hydrocarbons. Conversion of coal to liquid form and removal of ash are relatively straightforward operations. Efficient transformation of asphaltenes to lower hydrocarbons is a more difficult problem and represents the rate-controlling step in catalytically-promoted desulfurization, denitrogenization, and thence hydrogenation as well as thermal cracking of coal. The presence of asphaltenes, however, does not prevent conversion of coal into a liquid or readily liquefiable fuel oil useful for firing boilers and the like. A process termed the Synthoil process for converting coal to a low sulfur fuel oil is described in U.S. Pat. No. 3,840,456, the disclosure of which is hereby incorporated by reference. In the Synthoil process a coal-oil slurry is preheated in a preheater and cycled through a fixed catalytic bed reactor at a temperature in the range of 350.degree.-500.degree. C. under a hydrogen pressure ranging from 500-40,000 psig at a velocity substantially above turbulent flow.
A portion of the slurry issuing from the catalyst bed is recovered as the desired low sulfur (less than 0.2 weight percent sulfur) fuel oil product, while the remainder is recycled to the preheater or directly back to the catalytic reactor.